Contemporary aircraft may include an auxiliary power unit (APU) in addition to main propulsion engines. The APU may perform a variety of functions including providing secondary power to the aircraft as well as providing pressurized bleed air for main engine starting and the aircraft's environmental control system. Currently, airlines and maintenance personnel wait until a fault or problem occurs with the APU and then attempt to identify the cause and fix it during either scheduled or, more likely, unscheduled maintenance.